Electroplating of tin



Patented Jan. 11, 1949 ELECTROPLATING F TIN Frederick A. Lowenheim, Plainiield, N. 1., asaignor to Metal & Thermit Corporation, New York, N. Y., a corporation of New Jersey Application April 27, 1944, Serial No. 532,932

8 Claims.

This invention relates to the improvement of aqueous alkaline stannate electroplating processes. It also relates to improving the anode efilciency of alkaline stannate tinplating cells. It also relates to anodes of greater efliciency in aqueous alkaline stannate electroplating baths.

In electroplating tin from alkaline stannate baths, those who practice the art have employed soluble anodes of pure tin and have regulated the variables such as the composition of the bath, the cathode current density, the anode current density and the caustic concentration so that the solution of tin from the anodes replenishes the tin content of the bath as the tin is deposited at the cathode. Those attempts are successful only within limits and can be used to produce only limited flexibility in control of the process.

Attempts have been made to improve the efficiency of such baths by increasing the free alkali content of the solution, but this has been accompanied by a lowering of the cathode efficiency. To counteract this lowered cathode efiiciency the tin content of the bath has been raised. This series of measures does in fact permit the use of higher current densities, but they are accompanied by the disadvantage that they lead to the use of unduly high concentrations and to dragout losses which are uneconomical.

While the foregoing measures are effective to the extent indicated in connection with potassium stannate baths they are not useful for sodium stannate baths because of their limited cathode efliciency, the limited solubility of sodium stannate, and the much greater tendency of sodium stannate baths to precipitate insoluble tin compounds by hydrolysis.

High rates of cathode deposition have also been secured by increasing the ratio of anode area to cathode area, so that the cathode current densities remain high and the anode current densities remain relatively low, each of which is in the interests of emciency. This modification has a limitation imposed upon it by geometrical configuration and mechanical design, for example in the continuous electrotinning of strip steel wherein the mechanical features of the apparatus limit the area of anode.

It is an object of my invention to provide an anode for the electrolytic deposition of tin which will exhibit higher current efficiency than an anode of pure tin in any alkaline stannate plating bath without introducing harmful ingredients in the bath.

A second object of my invention is to increase the anode efllciency in alkaline stannate electroplating baths.

A third object of my invention is to improve the overall efliciency of potassium stannate electroplating cells and of sodium stannate electroplating cells. I

A fourth object of my invention is to form tin anodes that will be of increased efficiency in electroplating cells.

The objects of the invention are accomplished generally speaking by making an anode composed of tin alloyed with a small amount of aluminum and using it in alkaline stannate electroplating cells, particularly in potassium stannate electroplating cells.

The objects and nature of the invention will be more fully understood from a consideration of the specification and claims which follow:

The invention has particular application to the tinplating of strip steel, which is carried out by passing a steel strip through a plating cell as a moving cathode. In that application of the invention the speed of electrodeposition is of the essence. In one important aspect of that process, a large tank is provided with a series of anodes having faces relatively close together, and an aqueous alkaline stannate bath. Between the faces of the anodes the moving steel strip, connected in the cell as a cathode, is passed at a rate such that when it emerges from the cell it is perfectly plated. In order that this plating shall not vary in quality it is necessary that the bath operate with uniformity. For most economical results it is also desirable that the steel shall travel with the high speed offered by the cathode efliciency of a potassium stannate bath, but in some cases this cannot be continuously maintained economically because of the relative inefliciency of the anodes. It is economically impractical to alter the design of the cells to accommodate anodes of suiilcient oversize to achieve any satisfactory improvement in operating conditions. When it is realized that each of the anodes used in such cells "weighs in the thousands of pounds, this will be readily understood. By the practice of my invention the foregoing difficulties are overcome.

'According to my invention as applied to strip plating with an electrolytic bath containing alkali stannate and alkali hydroxide 'in selected and efficient concentration, the anodes are placed in the bath and connected to a source of current, the steel strip which is to be plated is passed between the anodes, with its surfaces as short distance from the faces 3 of the anodes, and is connected to the negative pole of the source of current. The current is then turned on and the current density increased until that critical point is reached at which a great increase in voltage is employed in order to film the anodes. After the anodes have been filmed the voltage and current density are reduced to proper operating conditions. The speed of the cathode through the bath is then adjusted to the highest speed which can be efliciently employed, considering the weight of tin it is desired to deposit on the surface of the cathode. The temperature of the bath. the current density on the anode, the curent density on the cathode. and the concentration of alkali hydroxide and tin in the bath are maintained at the most favorable figures.

If such a cell be operated with pure tin anodes, and an identical cell be operated under identical conditions but using my improved anodes, it will be found that practicable working anode current densities in the case of the improved anodes are almost doubled in many instances and in all cases are substantially increased. Speeds at which satisfactory plating can be carried out are inincreased in proportion.

The tin-aluminum alloys may be preparedby making a master alloy according to a selected formula, containing. for instance, 90% tin and aluminum. When tin and aluminum are melted and uniformly mixed so that the aluminum is evenly distributed in the mass a substantially perfect alloy is produced which may be subdivided to make the anodes. In making the anodes, calculated amounts of this master alloy are introduced into a melt of pure tin and cast into the desired form of the: anode. As little as one-tenth of one per cent of aluminum when added to the pure tin of the anode has a markedly beneficial eflect on the anode efficiency. The anodeefiiciency is further increased by increasing its aluminum content toward 1%. Anodes containing more than about 1% of aluminum also show beneficial efiects but the increase of aluminum content beyond about 1% produces little increase in efliciency.

The forgoing technique of alloy manufacture is satisfactory but not essential, since the anode can be prepared by melting pure tin and mixing with it the percentage of aluminum desired in the anode.

In order todemonstrate the improvement which is attained by this invention the reports of certain experiments areset forth hereinafter, and the results are plotted on the graphs in the accompanying drawings. In the drawings the graphs show anode efiiciency in per cent as the vertical axis and anode current density in amperes per square foot as the horizontal axis. Electrode efiiciency is the ratio of actual metal dissolved or deposited compared with the metal theoretically dissolved or deposited according to Faraday's law.

Fig. 1 shows in the upperline the emciency of a.

tin anode containing 0.72% aluminum, and in the lower line the relative efliciency of a pure tin anode. v

Fig. '2 is a similar graph showing in the top line the efiiciency of several anodes containing Example 1 .s s

Figure 1 shows the results of tests onpure tin and on 0.72% aluminum anodes respectively in a potassium stannate bath of the following composition maintained at 00 0.:

Sn 40.5 8J1. (.34MZ/l.) 7

Free KOH 11.0 g./l. (0.2 M./l.)

The improvement made possible'by the use of the aluminum alloy is brought out by the following comparative table showing the anode current densities obtainable for a given anode efllciency as read oi! the curves:

A B Eiliclency Anode Cur- Density Ratio: 2 rent Pure Sn 0.72% Al A Per cent On the average, the table shows that the anode current density may be more than doubled by the use of 0.72% aluminum alloyed with the tin.

Figure 2 shows the effect that changing the aluminum content of the anode has on the anode eificiency in a potassium stannate bath of the same composition as used' above.

Similar improvement has been observed in baths of other concentrations and temperatures.

Example 2 Tests of the new anodes were also run in sodium stannate baths, which were also improved thereby. The results shown in the following table were obtained with anodes containing 0.5% aluminum in a sodium stannate bath of the following composition at 0.:

The improvement in the sodium stannate bath is shown to be approximately 3540% omitting the first line of the table.

Example 3 The following four tests show that the addition of aluminum increases the anode eificiency regardless of the solution compositions. All tests were run at 90 0.:

(a) Solution:

Sn 31 g./l. (.26 M./l.) Free KOH 8.5 g./l. (.15 Mill.)

A B n Eiildency Cur- Density W rent Pure Sn 0.02% .u A

Per cent (b) Solution:

Sn 41 g./1. (.35M./1.) Free KOH 20 g./1. (.36 M./l.l

A B B Eflficiency Anode Cur- Density Ratio: I rent Pure Sn 32% Al Per cent (0) Solution:

Sn 41 g./l. (.35 MJI.) Free KOH 29 g./l. (.52 M./l.)

A B B Efliciency Anode Cur- Density Ratio:

rent Pure Sn 1.28% A1 A Per cent (d) Solution:

Sn 81 g./l. (.68 MJI.) FreeKOH 46 g./l. (.87 M./l.)

A B B Efliciency Anode Cur- Density Ratio: neat Pure 8n .wz, A] A Per cent Example 4 Tests were run on a solution having the renowins composition:

Sn s1 g./l. (.52 M./l.) Free non as g./l. (.64 MJL) Temperature 90 C.

In one tank, tin anodes in the shape of rods were hung and in the other, tin-aluminum (0.6% aluminum) alloy anodes of the same shape.

Tank 1-Sn anodes:

Average amperes, Total ampere hrs. 1450 Average anode current density, 68 A/SF Average anode area, 1.25 sq. it. Overall anode ei'iiciency.

6 Tank 2--0.6% Al anodes:

Average amperes, 1 Total ampere hrs. 1680 Average anode current density, A/SF- -Average anode area, 0.85 sq. it.

Overall anode eillciency. 89%

The results of Example 4 are somewhat more favorable than the results of Examples 1-3 both for the pure tin and for the aluminum alloyanodes, but this is explained by the fact that in Example 4 rod anodes were used, which tend to show better corrosion than the ilat anodes used in the other tests. Furthermore, the solution was agitated, while in the Examples 1-3 it was not. It will be observed that the overall anode efliciencies are almost equal, being 90 and 89% respectively, but that the average current density on the improved anode was 110 amperes per square foot as against 68 amperes per square foot for the tin anodes.

-While aluminum has no full equivalent, gallium produces a definite improvement when alloyed with tin. Manganese-tin alloys have also shown improved efilciency as anodes in alkaline stannate plating baths. These'anodes have also shown improved stability of the anode film in operation, being in this respect superior to pure tin. tin-aluminum, or tin-gallium anodes. Certain other elements including magnesium, sodium and bismuth showed nominal increases in emciency when alloyed with tin, but had other characteristics that were deleterious.

Example 5 1% gallium-tin alloy was prepared by melting together appropriate amounts of gallium and tin. The melt was cast as a plate anode and tested in a potassium bath corresponding to that of Example 1. The eilects of gallium were found to be beneficial but not to the same extent as aluminum.

Example 6 A 1.63% manganese-tin alloy was prepared from a master alloy containing 8% manganese. Anodes were tested in a potassium bath corresponding to that of Example 1. The anode current densities obtainable at a given efficiency were found to be from 25% to 65% higher than for pure tin.

An advantage of the invention is that the ratio of the critical anode current density to the working anode current density is smaller for the anodes of aluminum-tin, gallium-tin, and manganese-tin alloys than for pure tin anodes, which reduces the amount of excess electrical generating capacity needed for filming the anodes.

It is an incidental benefit of my invention that the mechanical properties of tin, such as the tensile strength and Brinell hardness, are considerably increased by the admixture of these small amounts of aluminum.

The invention has been described in its application to strip plating, but it is equally appiica-. ble to still tank plating, sbarrel plating, and automatic conveyor plating.

As many apparently widely different embodiments of this inventioin may be made without departing from the spirit and scope thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

What I claim is;

1. In the process of electroplating tin by passin: an electric current from an anode to cathode 1+ through an aqueous alkaline stannate' bath the step of improving the anode eiflciency of the bath which comprises employing an anode consisting essentially of tin alloyed with about 0.1 to 2% o! a metal of the class consisting of aluminum, gallium and manganese.

2. Process according to claim 1 in which the bathis an aqueous potassium stannate loath.-

3. Process' accordink to claim 1 in which the bath is an aqueous sodium stannate bath.

4. Process according to claim 1 in which said metal is aluminum.

5. Process according to claim 1 in which said metal is gallium.

6. Process according to claim 1 in which said metalismanganese.

7. In the process at electroplating tin by passing an electric current from an anode to a cathode through an aqueous potassium stannate bath. the step of improving the anode emciency of the'bath which comprises employing an anode consisting essentially of tin alloyed with about 1% aluminum.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1366.126 Fink; Aug. 28, 1923 2,078,868 Oplinger Apr. 27, 1837 OTHER REFERENCES Aui bau der zweistofllegierungen, by Hansen, pages 153-155, 1936.

Transactions American Electrochemical Society, vol. 45, pages 431-440, 1924. 

